A major portion of the worldwide petrochemical industry is concerned with the production of light olefin materials and their subsequent use in the production of numerous important chemical products via polymerization, oligomerization, alkylation and the like well-known chemical reactions. Light olefins include ethylene, propylene and mixtures thereof. These light olefins are essential building blocks for the modern petrochemical and chemical industries. The major source for these materials in present day refining is the steam cracking of petroleum feeds. For various reasons including geographical, economic, political and diminished supply considerations, the art has long sought a source other than petroleum for the massive quantities of raw materials that are needed to supply the demand for these light olefin materials. In other words, the holy grail of the R & D personnel assigned to work in this area is to find a way to effectively and selectively use alternative feedstocks for this light olefin production application, thereby lessening dependence of the petrochemical industry on petroleum feedstocks. A great deal of the prior art's attention has been focused on the possibility of using hydrocarbon oxygenates and more specifically methanol or dimethylether (DME) as a prime source of the necessary alternative feedstock. Oxygenates are particularly attractive because they can be produced from such widely available materials as coal, natural gas, recycled plastics, various carbon waste streams from industry and various products and by-products from the agricultural industry. The art of making methanol and other oxygenates from these types of raw materials is well established and typically involves the use of one or more of the following procedures: (1) manufacture of synthesis gas by any of the known techniques typically using a nickel or cobalt catalyst in a steam reforming step followed by the well-known methanol synthesis step using relatively high pressure with a copper-based catalyst; (2) selective fermentation of various organic agricultural products and by-products in order to produce oxygenates; or (3) various combinations of these techniques.
Given the established and well-known technologies for producing oxygenates from alternative non-petroleum raw materials, the art has focused on different procedures for catalytically converting oxygenates such as methanol into the desired light olefin products in order to make an oxygenate to olefin (OTO) process. These light olefin products that are produced from non-petroleum based raw materials must of course be available in quantities and purities such that they are interchangeable in downstream processing with the materials that are presently produced using petroleum sources. Although many oxygenates have been discussed in the prior art, the principal focus of the two major routes to produce these desired light olefins has been on methanol conversion technology primarily because of the availability of commercially proven methanol synthesis technology. A review of the prior art has revealed essentially two major techniques that are discussed for conversion of methanol to light olefins (MTO). The first of these MTO processes is based on early German and American work with a catalytic conversion zone containing a zeolitic type of catalyst system. Representative of the early German work is U.S. Pat. No. 4,387,263 which was filed in May of 1982 in the U.S. without a claim for German priority. This '263 patent reports on a series of experiments with methanol conversion techniques using a ZSM-5 type of catalyst system wherein the problem of DME by-product recycle is a major focus of the technology disclosed. Although good yields of ethylene and propylene were reported in this '263 patent, they unfortunately were accompanied by substantial formation of higher aliphatic and aromatic hydrocarbons which the patentees speculated might be useful as an engine fuel and specifically as a gasoline-type of material. In order to limit the amount of this heavier material that is produced, the patentees of the '263 patent propose to limit conversion to less than 80% of the methanol charged to the MTO conversion step. This operation at lower conversion levels necessitated a critical assessment of means for recovering and recycling not only unreacted methanol but also substantial amounts of a DME intermediate product. The focus then of the '263 patent invention was therefore on a DME and methanol scrubbing step utilizing a water solvent in order to efficiently and effectively recapture the light olefin value of the unreacted methanol and of the intermediate reactant DME.
This early MTO work with a zeolitic catalyst system was then followed up by the Mobil Oil Company who also investigated the use of a zeolitic catalyst system like ZSM-5 for purposes of making light olefins. U.S. Pat. No. 4,587,373 is representative of Mobil's early work and it acknowledged and distinguished the German contribution to this zeolitic catalyst based MTO route to light olefins.
Primarily because of an inability of this zeolitic MTO route to control the amounts of undesired C4+ hydrocarbon products produced by the ZSM-5 type of catalyst system, the art soon developed a second MTO conversion technology based on the use of a non-zeolitic molecular sieve catalytic material. This branch of the MTO art is perhaps best illustrated by reference to UOP's extensive work in this area as reported in numerous patents of which U.S. Pat. Nos. 5,095,163; 5,126,308 and 5,191,141 are representative. This second approach to MTO conversion technology was primarily based on using a catalyst system comprising a non-zeolitic molecular sieve, generally a metal aluminophosphate (ELAPO) and more specifically a silicoaluminophosphate molecular sieve (SAPO), with a strong preference for a SAPO species that is known as SAPO-34. This SAPO-34 material was found to have a very high selectivity for light olefins with a methanol feedstock and consequently very low selectivity for the undesired corresponding light paraffins and the heavier materials. This ELAPO catalyzed MTO approach is known to have at least the following advantages relative to the zeolitic catalyst route to light olefins: (1) greater yields of light olefins at equal quantities of methanol converted; (2) capability of direct recovery of polymer grade ethylene and propylene without the necessity of the use of extraordinary physical separation steps to separate ethylene and propylene from their corresponding paraffin analogs; (3) sharply limited production of by-products such as stabilized gasoline; (4) flexibility to adjust the product ethylene-to-propylene weight ratios over the range of 1.5:1 to 0.75:1 by minimal adjustment of the MTO conversion conditions; and (5) significantly less coke make in the MTO conversion zone relative to that experienced with the zeolitic catalyst system.
The classical OTO technology produces a mixture of light olefins primarily ethylene and propylene along with various higher boiling olefins. Although the classical OTO process technology possesses the capability of shifting the major olefin product recovered therefrom from ethylene to propylene by various adjustments of conditions maintained in the reaction zone, the art has long sought an oxygenate to propylene (OTP) technology that would provide better yields of propylene relative to the classical OTO technology. The driving force for this shift in emphasis towards propylene is the growth rate of the propylene market versus the growth rate of the ethylene market. The existing sources of propylene production in the marketplace are primarily based on conventional steam cracking of naphtha, LPG streams, propane streams and the like. Another major existing source of propylene is the olefins that are produced in a fluid catalytic cracking (FCC) hydrocarbon conversion process in the modern day refinery. Because the basic raw material used in an OTO process is derived from natural gas which is widely available particularly in remote areas of the world but unfortunately markets for this gas are typically not conveniently available near the location of the remote gas fields. These remote gas fields tend to be quite large and because of the relatively well-developed technology for converting natural gas to methanol and other oxygenates are viewed by those skilled in this art and being the next logical source of large-scale propylene production provided a commercially acceptable OTP process can be established which possesses intrinsic high selectivity for the desired propylene product.
Workers at Lurgi Oel Gas Chemie GmbH have recently announced a new fixed bed methanol to propylene (MTP) process which according to Lurgi offers the potential to satisfy the art's thirst for a propylene selective OTO type of process. It appears that the basic flow scheme and technical details of the Lurgi process offering in this field have been disclosed in U.S. Pat. No. 7,015,369 and describes a process for selectively producing propylene from a feedstock which comprises methanol and/or DME. Analysis of the two figures attached to this patent publication make it clear that Lurgi contemplates a reactor flow configuration for the oxygenate to propylene (OTP) synthesis portion of its flow scheme wherein three reactors are utilized with a steam diluent and fixed beds of oxygenate conversion catalysts in a parallel flow arrangement with respect to the oxygenate feed. The reactors are connected in a serial flow arrangement with respect to the effluents of the first reactor and the second reactor. The dual function OTP catalyst system taught as being useful in this flow scheme is rather narrowly defined in paragraph [0005] of this patent publication as a pentasil-type (i.e. ZSM-5 or ZSM-11 type) having an alkali content less than 380 ppm and a zinc oxide content of less than 0.1 wt-% coupled with a restriction on cadmium oxide content of the same amount. The teachings with respect to this catalyst are derived from a European patent, EP 0448000, filed by Sud Chemie and Lurgi claiming priority from an original German application that was filed in March of 1990. Thus the catalyst contemplated for use in Lurgi's flow scheme is well known to those skilled in this art. According to Lurgi's marketing presentation, the on-stream portion of the process cycle for this MTP process is expected to be 500 to 700 hours before in situ regeneration becomes necessary. (See Rothaemel et al. “Demonstrating the New Methanol to Propylene (MTP) Process” presented to the ERTC Petrochemical Conference in March of 2003 at Paris, France). The activity-stability of the MTP catalyst in this Lurgi presentation show a significant drop in conversion activity over the first five cycles with each cycle being terminated after the oxygenate conversion level drops to about 94% to 95% of the oxygenate feed. Lurgi also contemplates that at the end of the cycle when the conversion has dropped to a level of about 94% of the equivalent in the feed that the reactors will be shut down and the catalyst regenerated in situ using an air/nitrogen mixture to burn off the detrimental coke deposits.
In order to substantially improve the fixed bed OTP technology of the prior art, UOP has recently embarked on a program to apply classical moving bed technology to the fixed bed OTP process of the prior art. The term “moving bed technology” is well understood by those of ordinary skill in the chemical engineering art to mean that particles of the OTP catalyst move through the reaction zone as well as the associated regeneration zone in a compact, non-fluidized bed driven primarily by the action of gravity. As part of this program UOP has focused on using moving bed regeneration technology to regenerate one or more of the known OTP catalyst systems of the prior art. These OTP catalyst systems are required in the prior art processes such as the one proposed by Lurgi to be dual-functional in the sense that they must be able to catalyze both the OTP reactions and the olefin interconversion reactions necessary to convert C2 and C4+ olefins to the desired propylene product. The known dual-function catalysts that can be applied to this OTP service are characterized primarily by the presence of a suitable molecular sieve and the absence of a metallic functionality. This lack of a metallic functionality leads to the problem addressed by the present invention when an attempt is made to apply the commercially proven annular moving bed regeneration technology to coked catalysts having a composition similar to such an OTP catalyst system. UOP's contributions to the art of moving bed regeneration technology started at least as early as 1972 with the issuance of U.S. Pat. No. 3,652,231 which disclosed two versions of a novel apparatus for use in regenerating coke-containing catalyst systems. In particular the annular moving bed system shown in the apparatus of FIG. 3 of this '231 patent soon became the benchmark for the moving bed regeneration art in the petroleum conversion industry. The teachings of this '231 patent with respect to the type of catalyst systems that could be regenerated therein are however limited in every case to a catalyst system that contains a metallic functionality that is a known CO oxidation promoter and thus the problem addressed by the present invention was not recognized at the time commercial embodiments of the moving bed regeneration art crystallized. This situation was also true in the subsequently issued apparatus patents with respect to the novel moving bed apparatus that was initially commercialized by UOP. These early apparatus embodiments can be found in U.S. Pat. Nos. 3,647,680 and 3,692,496. There were many subsequent improvements and embellishments made to this area of moving bed regeneration technology which can be found in, for example: U.S. Pat. Nos. 3,981,824; 4,094,814; 4,578,370; 5,034,117 and 6,133,183. All of these secondary patents as well as many others that could be cited addressed various methods for improving the performance of moving bed regeneration technology but in all cases the catalyst systems that were taught in this prior art for this application of moving bed regeneration technology also contained a metal functionality which acted as an inherent CO oxidation promoter. The art on the application of moving bed regeneration technology to coke-contaminated catalyst is thus focused to a large extent on the problem of regenerating a catalyst system that contains a metal functionality which under the conditions prevailing in the moving bed regeneration zone acts in the first instance to oxidize any CO products of the principal coke burning reactions to CO2 and thus the production of a hazardous flue gas stream from the moving bed regeneration apparatus described in this line of the prior art was not observed and recorded. It is thus clear that the application of moving bed regeneration technology as it has been perfected in the body of the prior art cited above did not identify the problem addressed by the present invention, much less provide a solution therefore.
UOP has diligently pursued the objective of substantially improving the fixed bed OTP technology contributed by Lurgi to the prior art using moving bed technology as the principal thrust of its creative effort. UOP filed a series of applications directed to inventions concerning the ramifications of the application of moving bed technology to this application focused primarily on the reactor side of the technology rather than the regeneration side. UOP's contributions in this area of the art include, inter alia, U.S. Pat. Nos. 7,371,915; 7,371,916; 7,405,337; and 7,408,092. All of the teachings of these UOP patents are specifically incorporated herein by reference.
During the course of UOP's investigation of the application of moving bed technology to the problem of regenerating coke-deactivated catalysts of the prior art, it soon became evident that there existed a problem that had not been adequately addressed in the prior art. In a nutshell the problem that surfaced during the course of this investigation was the fact that the application of moving bed regeneration technology as developed in the line of prior art discussed above to regeneration of coke-containing catalyst systems that do not contain a CO oxidation promoter produced an effluent flue gas stream that contained unacceptable and hazardous levels of CO. The problem then addressed by the present invention is to modify the commercially proven moving bed regeneration apparatus that is taught in this line of prior art to enable the production of an effluent flue gas stream therefrom which does not contain hazardous levels of CO when it is applied to the regeneration of coke-containing catalysts that do not contain a CO oxidation promoter such as, for example, OTP catalysts.
Careful investigation of the origin of the undesired CO by-product production in the combustion zone of a moving bed regenerator when it is used to regenerate coke-containing catalysts like the dual-functional OTP catalyst of primary interest to the present invention has led the present inventor to discern that the problem is primarily attributable to the absence of a metallic functionality in these catalyst systems such as the dual-function OTP catalyst systems. The present invention thus envisions a solution to this problem of production of a flue gas stream containing undesired and hazardous amounts of CO from conventional moving bed regeneration apparatus by modifying the moving bed regeneration apparatus characterized in the line of prior art analyzed above by integrating a CO oxidation zone into the flue gas collection zone of the preferred annular moving bed regeneration apparatus disclosed therein in order to eliminate the CO hazard by selectively and catalytically oxidizing CO to CO2 with a portion of the unreacted oxygen withdrawn from the coke combustion zone of the moving bed regeneration apparatus. The principal advantages associated with this solution to this hazardous flue gas problem are: 1) the integral oxidation zone functions autogenously to produce an acceptable effluent flue gas stream without the necessity of any particular command and control provisions; 2) there is no necessity to contaminate the coke-containing dual-function catalyst that is being subjected to regeneration with an undesired CO oxidation promoter that can compromise or inhibit its performance when it is returned to the reactor side of the unit; 3) there is no risk of environmental contamination since the hazardous material does not leave the modified apparatus; and 4) if a portion of the resulting effluent flue gas stream is used as a diluent in the combustion gas stream charged to the coke combustion zone of the regeneration unit then the coke combustion reactions are not inhibited as they would be by the presence of substantial amounts of CO in the absence of the present invention.